The spreading of wettened spreadable material has the advantage that the wettened spreadable material adheres better and remains adhered to the place where it falls. It is thus possible for preventive spreading of a road surface to be carried out with wettened road salt, wherein the wettened road salt adheres to the road surface and is not blown away, while the forecast ice formation only occurs some time later.
In spreading with wettened material it is essential that the supply of liquid wetting agent is adapted to the variable metering of spreadable granular material. In spreading of road salt the delivery is thus dependent on the speed of the vehicle on which the spreading apparatus is placed, which speed is measured with a measuring wheel which also serves to energize the hydraulic system.
According to known prior art liquid wetting agent is supplied via a pump, wherein the flow rate of the pump depends on the delivery or metering of the spreadable granular material. In order to avoid the reservoir of liquid wetting agent draining through the pump it is necessary for the feed conduit which supplies wetting agent from the pump to the spreader members to be provided with a biased non-return valve which must have for instance an additional seat pressure of about 1.5 bar.